Permanent magnets are used in various areas such as microelectronics, automobiles, medical devices, power generation, and the like. Permanent magnets are typically formed from hard magnetic materials which also find applications in the automotive, aerospace and telecommunication industries. Rare earth magnets, such as Nd—Fe—B, have a higher energy density than most other hard magnets. Moreover, such magnets are used in automotive applications such as starters, small motors, alternators, sensors, meters, and electric and hybrid vehicle propulsion systems.
Sintered rare earth magnets are usually made from powder metals by forming to shape under pressure and then sintering. The overwhelming majority of hard magnets are formed from ferrite and Nd—Fe—B. Ferrite is less expensive but with only modest magnetic properties. This material is mainly used in applications where size and weight are not main design considerations.
The intrinsic properties necessary for high strength permanent magnets include a high saturation magnetization, large magnetocrystalline anisotropy, and a reasonable high Curie temperature. These properties are strongly influenced by the factors such as microstructure, chemical composition, magnet size, surface coating etc. The material properties (e.g. the magnetic properties) that are influenced by the microstructure include phases and composition distribution, grain size, grain morphology, and orientation, as well as grain boundaries. When the grain size is below a critical limit known as the single domain limit, demagnetization is much more difficult, leading to excellent hard magnetic properties. The single domain limit is related to specific intrinsic magnetic properties, including the anisotropy constant and the saturation magnetization. For Nd—Fe—B magnets, the single domain limit is about 300 nm.
The preferred commercial technique to generate a fine-scale microstructure is melt spinning. Depending on the processing parameters, melt spinning generates a microstructure that includes fine, equiaxed grains on the order of 20-30 nm or an amorphous structure. It is critical to retain a fine microstructure upon further processing to optimize the magnetic properties. Anisotropic magnets are produced with grains in preferred crystallographic alignment. A high degree of crystallographic alignment results in high energy products. The degradation in the microstructure, and the limited crystallographic alignment achievable, limits commercially available energy products to about 50 megagauss-oersteds (MGOe), comparing to the theoretical maximum of 64 MGOe.
Sintered Nd—Fe—B permanent magnets have very good magnetic properties at low temperatures. After magnetization, permanent magnets are in a thermodynamically non-equilibrium state. Any changes in the external conditions, in particular the temperature, result in a transition to another more stable state. These transitions are typically accompanied by changes in the magnetic properties. Due to limited low Curie temperature of the Nd2Fe14B phase, the magnetic remanence and intrinsic coercivity decrease rapidly with increased temperature. There are two common approaches for improving the thermal stability of Nd—Fe—B permanent magnets and for increasing magnetic properties in order to obtain compact, lightweight, and powerful motors for hybrid and electrical vehicles. One approach is to raise the Curie temperature by adding Co, which is completely soluble in the Nd2Fe14B phase. However, the coercivity of the Nd—Fe—B magnets with Co decreases, possibly because of the nucleation sites for reverse domains. The second approach is to add heavy rare-earth elements. It is known that the substitution of dysprosium (Dy) for neodymium (Nb) or iron (Fe) in Nd—Fe—B magnets results in increases of the anisotropic field and the intrinsic coercivity, and a decrease of the saturation magnetization (C. S. Herget, Metal, Poed. Rep. V. 42, P. 438 (1987). W. Rodewald, J. Less-Common Met., V111, P 77 (1985). D. Plusa, J. J. Wystocki, Less-Common Met. V. 133, P. 231 (1987)). It is believed that once a nucleus of reversed domain appears at the surface of the grain, magnetic reversal of the whole grain occurs immediately. Reverse magnetic domain only comes from the grain boundary. If we can make Dy uniformly distributed around the grain boundary, the coercivity should be increased, and the remanence should not change much. Therefore, it is a common practice to add the heavy rare-earth metals such as Dy or Tb into the mixed metals before melting and alloying. However, Dy and Tb are very rare and expensive. In the nature, very small fraction of rare-earth metals are heavy HEs, and heavy REs contain only about 2-7% Dy. The price of Dy has increased sharply in recent times. Tb is needed if even higher magnetic properties are required, and it is much more expensive than Dy.
The ideal microstructure for sintered Nd—Fe—B based magnets is Fe14Nd2B grains perfectly isolated by the nonferromagnetic Nd-rich phase (a eutectic matrix of mainly Nd plus some Fe4Nd1.1B4 and Fe—Nd phases stabilized by impurities). The addition of Dy and/or Tb leads to the formation of different ternary intergranular phases based on Fe, Nd and Dy or Tb. These phases are located in the grain boundary region and at the surface of the Fe14Nd2B grains.
Any addition of elements to improve the magnetic property should fulfill the following conditions: 1) the intermetallic phase should be nonferromagnetic to separate the ferromagnetic grains; 2) the intermetallic phase must have a lower melting point than the Nd2Fe14B phase to produce a dense material via liquid phase sintering; and 3) the elements should have a low solubility in Nd2Fe14B to keep good magnetic properties. The coercivity is known to be greatly influenced by the morphology of the boundary phases between Nd2Fe14B grains.
Accordingly, there is a need for improved methods of making permanent magnets, and in particular, Nd—Fe—B based magnets